First of all check the motherboard's manual to see if you can disable the onboard vga by using jumber settings. If so, open the chassis and do it. If not, check the bios to see if there is a choice for you to disable it by using bios settings.
If not, put the card on and power on the computer. Under Windows' Control Panel enter
System and Device Manager. Check how many vga cards are present in your computer.
If two, disable the old one and leave the new one enabled. If not, check for new hardware or conflicts between vga cards.